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Hastings makeup air unit being used as unvented hanging heater?

Normally residential tech venturing into the commercial forum here on hvac-talk for the first time due to running a commercial call today.

In addition to the simple moving of a thermostat and placing it in a lockable cover, they asked me to take a look at their beast of a Hastings heater. Well, after I consulted my company owner, who also happens to be my dad, about it since he has eons of commercial experience, he informed me that what I was being told was a large hanging warehouse heater is actually a makeup air unit. It's two large intake ducts are just open now as the connections that used to go through the roof have been eliminated years ago. The large amount of exhaust/vent air that used to be in the building is gone as well. They have been using this 836,000 btu beast for at least the past couple of years as a large warehouse heater heating about 8000 sq. ft. with 20 foot ceilings. And in normal Hastings fashion the unit does not vent out exhaust, it is just moving its air over open flame.

I wanted to double-check what my dad left them with which is that this unit is only designed to be used as a makeup air unit and if it is no longer pulling its intake air from outside and running along with adequate building exhaust then it is dangerous to use.

Would you agree with this or do these units run so clean that there is no danger of carbon monoxide and other combustion byproducts building up in this now not very well ventilated space?

indeed your dad is right this unit is a make up air and is intended to be used as one hence MAKE UP AIR meaning it pulls fresh air in to the building these units must be run with adequate exhasut fan's that should be interlocked with the make up air unit when it comes on.

Thank you very much. The 2nd opinion puts me more at ease. Telling a customer that something they have been using for heat is unsafe and needs to be immediately disabled is a big step and since I was not familiar with makeup air units I was a little uncomfortable doing that without more information, but now that I have learned a little about them I realize that you, and my old man, are correct.

With no exhaust and no fresh air it's wrong. Although it happens more then one might think. MAUs are not heaters. Not intended to be heaters. Intended to change the air in a space in cahoots with exhaust.

Hopefully this statement will help you remember the difference; if the flame touches the air going into the building, it must be taken from outside and exhausted outside immediately. Recirculating space heating systems will have an exhaust and a heat exchanger to separate the exhaust gases from the indoor air. Makeup heaters can exhaust into the air steam but must be taking in outside air and exhausting the air after a single pass (kitchen hood tempering heater, garage bay tempering with direct exhaust, ect).

Hopefully this statement will help you remember the difference; if the flame touches the air going into the building, it must be taken from outside and exhausted outside immediately. Recirculating space heating systems will have an exhaust and a heat exchanger to separate the exhaust gases from the indoor air. Makeup heaters can exhaust into the air steam but must be taking in outside air and exhausting the air after a single pass (kitchen hood tempering heater, garage bay tempering with direct exhaust, ect).

Good Call and Very Good Catch!

I would have to disagree.
It's very common to have a direct fired unit with recirculating return air. But they run minimum 20% OA.

Never seen it myself, but the day I stop learning is the day I start pushing daisies. Wonder if it's a local code thing...

How does a recirculating direct flame system sense buildup of CO/CO2/toxic gasses due to problem with combustion? On unvented portable unit heaters. an oxygen deprivation sensor is a requirment... Inquiring minds would like to know.

Never seen it myself, but the day I stop learning is the day I start pushing daisies. Wonder if it's a local code thing...

How does a recirculating direct flame system sense buildup of CO/CO2/toxic gasses due to problem with combustion? On unvented portable unit heaters. an oxygen deprivation sensor is a requirment... Inquiring minds would like to know.

Still requires exhaust to so air is changed. Cannot be done as stand alone heat setup.